Your tankless unit ran fine for three years. Then one Tuesday morning the shower went cold in the middle of a rinse, and the display threw a code you have never seen. You did the classic homeowner thing: flipped the breaker, waited a minute, tried again. Still cold.

Here’s the good news. Most tankless failures are not the end of the unit. They’re a handful of predictable problems that show up in a predictable order, and you can often narrow yours down before anyone touches a wrench. You don’t need to be handy to do that. You need to know what the machine is telling you.

If you’re in the East Bay and you’ve already ruled out the simple stuff, Tankless Water Heater Repair in Pleasanton is a phone call away. But before you make it, spend ten minutes reading the symptoms. You’ll save yourself a diagnostic fee, and you’ll sound like you know what you’re talking about when the tech shows up.

What actually kills a tankless water heater

Tankless units are simple machines with a hard job: heat a lot of water, fast, on demand. They do it by pushing water through a copper heat exchanger while a gas burner or electric element fires underneath. Everything that goes wrong traces back to one of three things breaking that relationship.

It’s almost never the electronics. It’s almost always scale, airflow, or ignition. I’ve watched homeowners replace a control board when the real culprit was a mineral crust the thickness of a dime.

Mineral scale, the number one killer

Hard water leaves calcium and magnesium behind every time it passes through the heat exchanger. Those deposits bake onto the metal, insulate it, and force the unit to work harder to hit the same temperature. Over months, that turns into a shorter, hotter, less reliable hot water supply.

Where you live matters a lot here. According to the U.S. Department of Energy, water hardness varies widely across the country, and that variation directly affects how often a tankless unit needs descaling. If your area runs hard, you may need to flush the unit every year. Soft water areas can stretch it further.

Easy test for you: fill a clear glass, let it sit overnight, and look at the bottom in the morning. A chalky film means you’ve got minerals. That film is what’s forming inside your heat exchanger right now.

Error codes are your unit talking, so listen

Every major brand runs its own code system, which is why a code that means “flame failure” on one machine means something totally different on another. The one habit worth building is photographing the display the moment a code appears. Codes often clear themselves on reset, and then you’re describing a symptom to a stranger over the phone with nothing to point at.

Most codes cluster into three families: ignition and flame sensing, water flow and pressure, and exhaust or venting. That grouping alone tells you whether you’re looking at a gas problem, a plumbing problem, or a venting problem. You don’t need to decode the exact number to ask a smarter question.

Ignition and flame sensing problems

If the unit clicks, tries to fire, and gives up, you’re in ignition territory. Common causes include a dirty flame sensor, a clogged burner, or gas pressure that has drifted out of spec. On the electric side, a failed heating element plays the same role. Neither is usually a crisis, but both stop hot water cold. If your unit clicks three times and quits, stop resetting it. You’re not fixing anything; you’re just listening to it fail.

Water flow tells you more than you think

Tankless heaters need a minimum flow rate before they’ll fire at all. Below that threshold, the unit simply sits there while you stand in the shower wondering what happened. That’s why low hot water pressure and a dead unit often look identical from the showerhead.

Check the obvious first. Any shutoff valve that’s not fully open, a clogged inlet filter screen, or a shower cartridge gone bad can all starve the unit below its firing threshold. Pull the inlet filter on your unit and look at it. If it’s a furry little disc of grit, that’s your whole problem, and it cost you nothing to find.

Hard water scale makes this worse because it narrows the internal passages over time. This is where annual maintenance stops being optional. Skipping it doesn’t just shorten the unit’s life, it makes every other symptom harder to diagnose.

Venting, the failure nobody checks

A blocked or improperly sloped exhaust vent traps combustion gases and trips the unit’s safety shutoff. You’ll see this more in winter when nests, debris, or ice build up around the termination point. Look at where the vent exits the house. It should be clear and pitched correctly. If you can see daylight through it, that’s a good sign.

Never run a gas appliance with a known vent problem to “see if it clears up.” Carbon monoxide doesn’t announce itself. If you smell exhaust indoors or the unit shuts off every time it fires, stop using it and get it looked at.

A quick decision guide before you call anyone

Here’s the order I’d work through, and it takes about fifteen minutes. This is the framework I’d trust if it were my own house.

1. Check the breaker and gas valve. Sounds insulting, but a tripped breaker accounts for more “broken” units than anything else.
2. Look at the display and photograph it. Note whether the unit is firing, clicking, or silent.
3. Test one tap, not the shower. Run a kitchen faucet on hot only and time how long until it warms.
4. Pull and inspect the inlet filter screen. Clean it if it’s dirty. This is free.
5. Check the vent termination outside. Clear any blockage, but only at ground level and only if you’re comfortable.
6. Open the cold water and watch the flow. Weak flow at multiple fixtures points to a supply issue, not the heater.
7. Note the water hardness in your area. Hard water plus a year without a flush equals scale, almost every time.

If you get through that list and the unit still won’t fire, you’re past the DIY line. Burner cleaning, gas pressure checks, and heat exchanger descaling all require parts and tools most people don’t have, and a botched gas repair is genuinely dangerous. According to the Air-Conditioning, Heating, and Refrigeration Institute, proper installation and servicing by qualified technicians is what keeps these systems operating safely and efficiently over their lifespan.

One more note that most homeowners never think about. The Home Innovation Research Labs maintains the national standards that govern how homes are built and how their systems are expected to perform. Water heating sits squarely inside those standards, which is why cutting corners on a repair tends to show up again later.

When repair makes sense and when it doesn’t

Age is the deciding factor more often than people want to admit. Electric tankless units generally last around twenty years, gas units closer to ten to fifteen, and repairs get less worthwhile as you approach the end of that window. A single failed component on a five-year-old unit? Fix it. A third repair in two years on a fourteen-year-old machine? You’re paying rent on a problem.

Parts availability matters too. Some discontinued models leave you waiting weeks for a control board, and a week of cold showers has a cost that doesn’t show up on the invoice. My honest read: if the diagnosis comes back as scale or a dirty filter, repair is the obvious move and it’s cheap. If it’s a cracked heat exchanger, that’s the unit’s spine, and replacement is usually the better call.

What to do with all this

You started this because you wanted hot water, not a lecture on heat exchangers. Fair. The short version is that tankless failures come down to scale, flow, or ignition nearly every time, and the diagnostic order above will get you most of the way there for free. Handle the filter, watch the codes, get a yearly flush if your water runs hard, and you’ll dodge the majority of breakdowns.

When you’ve exhausted the easy checks and the water’s still cold, that’s not a failure on your part. It just means the problem lives behind a panel you shouldn’t open. Get someone qualified out, hand over your photos and your notes, and skip the part where you pay for guesswork.

What’s your unit actually doing? A code, a click, or total silence? Knowing which one puts you three steps ahead of the next person who calls.